diff --git a/src/Bitmap.c b/src/Bitmap.c
index 496a405b5..9c417949b 100644
--- a/src/Bitmap.c
+++ b/src/Bitmap.c
@@ -617,10 +617,13 @@ ReturnCode Png_Encode(Bitmap* bmp, struct Stream* stream, Png_RowSelector select
 
 	struct ZLibState zlState;
 	struct Stream chunk, zlStream;
-	uint32_t stream_len;
+	uint32_t stream_end, stream_beg;
 	int y, lineSize;
 	ReturnCode res;
 
+	/* stream may not start at 0 (e.g. when making default.zip) */
+	if ((res = stream->Position(stream, &stream_beg))) return res;
+
 	if (!selectRow) selectRow = Png_SelectRow;
 	if ((res = Stream_Write(stream, png_sig, PNG_SIG_SIZE))) return res;
 	Stream_WriteonlyCrc32(&chunk, stream);
@@ -666,13 +669,14 @@ ReturnCode Png_Encode(Bitmap* bmp, struct Stream* stream, Png_RowSelector select
 	/* Write end chunk */
 	Stream_SetU32_BE(&tmp[4],  0);
 	Stream_SetU32_BE(&tmp[8],  PNG_FourCC('I','E','N','D'));
-	Stream_SetU32_BE(&tmp[12], 0xAE426082UL); /* CRC32 of iend */
+	Stream_SetU32_BE(&tmp[12], 0xAE426082UL); /* CRC32 of IEND */
 	if ((res = Stream_Write(stream, tmp, 16))) return res;
 
-	/* Come back to write size of data chunk */
-	if ((res = stream->Length(stream, &stream_len))) return res;
-	if ((res = stream->Seek(stream, 33)))            return res;
+	/* Come back to fixup size of data in data chunk */
+	if ((res = stream->Length(stream, &stream_end)))   return res;
+	if ((res = stream->Seek(stream, stream_beg + 33))) return res;
 
-	Stream_SetU32_BE(&tmp[0], stream_len - 57);
-	return Stream_Write(stream, tmp, 4);
+	Stream_SetU32_BE(&tmp[0], (stream_end - stream_beg) - 57);
+	if ((res = Stream_Write(stream, tmp, 4))) return res;
+	return stream->Seek(stream, stream_end);
 }
diff --git a/src/Platform.c b/src/Platform.c
index cd6bd9a18..fae5d2b21 100644
--- a/src/Platform.c
+++ b/src/Platform.c
@@ -473,10 +473,10 @@ ReturnCode File_Open(FileHandle* file, const String* path) {
 	return File_Do(file, path, GENERIC_READ, OPEN_EXISTING);
 }
 ReturnCode File_Create(FileHandle* file, const String* path) {
-	return File_Do(file, path, GENERIC_WRITE, CREATE_ALWAYS);
+	return File_Do(file, path, GENERIC_WRITE | GENERIC_READ, CREATE_ALWAYS);
 }
 ReturnCode File_Append(FileHandle* file, const String* path) {
-	ReturnCode res = File_Do(file, path, GENERIC_WRITE, OPEN_ALWAYS);
+	ReturnCode res = File_Do(file, path, GENERIC_WRITE | GENERIC_READ, OPEN_ALWAYS);
 	if (res) return res;
 	return File_Seek(*file, 0, FILE_SEEKFROM_END);
 }
@@ -608,10 +608,10 @@ ReturnCode File_Open(FileHandle* file, const String* path) {
 	return File_Do(file, path, O_RDONLY);
 }
 ReturnCode File_Create(FileHandle* file, const String* path) {
-	return File_Do(file, path, O_WRONLY | O_CREAT | O_TRUNC);
+	return File_Do(file, path, O_RDWR | O_CREAT | O_TRUNC);
 }
 ReturnCode File_Append(FileHandle* file, const String* path) {
-	ReturnCode res = File_Do(file, path, O_WRONLY | O_CREAT);
+	ReturnCode res = File_Do(file, path, O_RDWR | O_CREAT);
 	if (res) return res;
 	return File_Seek(*file, 0, FILE_SEEKFROM_END);
 }
diff --git a/src/Platform.h b/src/Platform.h
index 3eccafc5e..fd6fc1c78 100644
--- a/src/Platform.h
+++ b/src/Platform.h
@@ -132,6 +132,7 @@ ReturnCode File_GetModifiedTime(const String* path, TimeMS* ms);
 ReturnCode File_SetModifiedTime(const String* path, TimeMS ms);
 
 /* Attempts to create a new (or overwrite) file for writing. */
+/* NOTE: If the file already exists, its contents are discarded. */
 ReturnCode File_Create(FileHandle* file, const String* path);
 /* Attempts to open an existing file for reading. */
 ReturnCode File_Open(FileHandle* file, const String* path);
diff --git a/src/Resources.c b/src/Resources.c
index aa9000149..82bcd31f3 100644
--- a/src/Resources.c
+++ b/src/Resources.c
@@ -244,6 +244,40 @@ static ReturnCode ZipPatcher_EndOfCentralDir(struct Stream* s, uint32_t centralD
 	return Stream_Write(s, header, 22);
 }
 
+static ReturnCode ZipPatcher_FixupLocalFile(struct Stream* s, struct ResourceTexture* e) {
+	String name = String_FromReadonly(e->Filename);
+	uint8_t tmp[2048];
+	uint32_t dataBeg, dataEnd;
+	uint32_t i, crc, toRead, read;
+	ReturnCode res;
+
+	dataBeg = e->Offset + 30 + name.length;
+	if ((res = s->Position(s, &dataEnd))) return res;
+	e->Size = dataEnd - dataBeg;
+
+	/* work out the CRC 32 */
+	crc = 0xffffffffUL;
+	if ((res = s->Seek(s, dataBeg))) return res;
+
+	for (; dataBeg < dataEnd; dataBeg += read) {
+		toRead = dataEnd - dataBeg;
+		toRead = min(toRead, sizeof(tmp));
+
+		if ((res = s->Read(s, tmp, toRead, &read))) return res;
+		if (!read) return ERR_END_OF_STREAM;
+
+		for (i = 0; i < read; i++) {
+			crc = Utils_Crc32Table[(crc ^ tmp[i]) & 0xFF] ^ (crc >> 8);
+		}
+	}
+	e->Crc32 = crc ^ 0xffffffffUL;
+
+	/* then fixup the header */
+	if ((res = s->Seek(s, e->Offset)))      return res;
+	if ((res = ZipPatcher_LocalFile(s, e))) return res;
+	return s->Seek(s, dataEnd);
+}
+
 static ReturnCode ZipPatcher_WriteData(struct Stream* s, struct ResourceTexture* tex, const uint8_t* data, uint32_t len) {
 	ReturnCode res;
 	tex->Size  = len;
@@ -254,6 +288,21 @@ static ReturnCode ZipPatcher_WriteData(struct Stream* s, struct ResourceTexture*
 	return Stream_Write(s, data, len);
 }
 
+/*static ReturnCode ZipPatcher_WriteStream(struct Stream* s, struct ResourceTexture* tex, struct Stream* src) {
+	uint8_t tmp[2048];
+	uint32_t read;
+	ReturnCode res;
+	if ((res = ZipPatcher_LocalFile(s, tex))) return res;
+
+	for (;;) {
+		res = src->Read(src, tmp, sizeof(tmp), &read);
+		if (res)   return res;
+		if (!read) break;
+
+		if ((res = Stream_Write(s, tmp, read))) return res;
+	}
+	return ZipPatcher_FixupLocalFile(s, tex);
+}*/
 static ReturnCode ZipPatcher_WriteStream(struct Stream* s, struct ResourceTexture* tex, struct Stream* src) {
 	uint8_t tmp[2048];
 	uint32_t read;
@@ -274,23 +323,15 @@ static ReturnCode ZipPatcher_WriteStream(struct Stream* s, struct ResourceTextur
 	}
 
 	tex->Crc32 = crc32.Meta.CRC32.CRC32 ^ 0xFFFFFFFFUL;
-	return 0;
+	return ZipPatcher_FixupLocalFile(s, tex);
 }
 
 static ReturnCode ZipPatcher_WritePng(struct Stream* s, struct ResourceTexture* tex, Bitmap* src) {
-	struct Stream crc32;
 	ReturnCode res;
 
-	res = ZipPatcher_LocalFile(s, tex);
-	if (res) return res;
-	//Stream_WriteonlyCrc32(&crc32, s);
-
-	//res = Png_Encode(src, &crc32, NULL, true);
-	res = Png_Encode(src, s, NULL, true);
-	if (res) return res;
-
-	//tex->Crc32 = crc32.Meta.CRC32.CRC32 ^ 0xFFFFFFFFUL;
-	return 0;
+	if ((res = ZipPatcher_LocalFile(s, tex)))   return res;
+	if ((res = Png_Encode(src, s, NULL, true))) return res;
+	return ZipPatcher_FixupLocalFile(s, tex);
 }